The embarrassment came for Twitter while it is in the middle of a class-action lawsuit for gender discrimination.
Fraternity parties, mostly known as frat parties, are usually organised by young college students to meet new people and have fun. Fraternities are houses where a bunch of male college-students live, so a frat party by definition is a party thrown by a fraternity.
The party was held on Tuesday by one of Twitter's San Francisco-based teams for their internal happy hour, and involved frat-style accoutrements such as a keg, red Solo cups and beer pong, according to a report by student magazine Fusion.
In recent years, referring to a tech company's culture as "fratty" is a way to describe Silicon Valley's gender disparities, and bemoan the male-dominated culture that results.
After a female Twitter employee posted a picture of the party on a Facebook group for women, it spread rapidly, with many highlighting it as the latest example of how Silicon Valley culture can be exclusionary towards women.
Twitter later apologised for the event.
"This social event organised by one team was in poor taste at best, and not reflective of the culture we are building here at Twitter," company spokesperson Jim Prosser was quoted as saying.
"We have had discussions internally with the organising team, and they recognise that this theme was ill-chosen," Prosser added.
A former engineer at Twitter, Tina Huang, has filed a class-action lawsuit in March this year claiming that the company's promotion and hiring procedures fail to ensure adequate diversity in high-level coding positions.
Only 10 percent of Twitter's tech employees are female, according to company data released last year.